In many surgical operations it is necessary to obtain direct access to the cranial cavity and the brain. To perform such operations, it is often necessary to drill holes through the skull bone. Since the bone is very hard, it is necessary to apply significant pressure to drill through it.
A very satisfactory cranial drill with an automatic clutch mechanism for disengaging the drill when it approaches the interior layers of the skull bone or as it breaks through the interior of the skull is described in U.S. Pat. No. 4,456,010, issued June 26, 1984, and assigned to the assignee of the present invention. The disclosure of that patent is fully incorporated by reference in this application.
The invention described in U.S. Pat. No. 4,456,010 incorporates a sleeve about the exterior of the drill for holding the varius parts of the drill together during use. It is described at Column 6, lines 38 through 46 that sleeve 70 of that patent includes a raised portion 74 extending circumferentially on sleeve 70, partially thereabout and is axially aligned with a recess 14 on drill body 10. Raised portion 74 may be deformed radially inwardly to engage recess 14 on drill body 10 to hold sleeve 70 and driver 50 on drill body 10. Although that arrangement works satisfactorily, it has been found that this prior drill can be improved by incorporating a retaining ring in slot 14 shown in the previous patent, assembling the sleeve 70 over drill body 10 and then affixing the retaining ring to the inside surface of the sleeve, preferably by means of ultrasonic welding.